Moving a certain part of the image

hi there.
Ive copy and pasted into an image and i thought i got it where i wanted so i saved it and exited the program. But then i realised that the image i pasted was a few pixels off.
I have been trying to select a portion of the image and move it to where i want it but cant find how. Ive selected the portion and clicked on move then selection but it moves the selection area not the image within it.
Please help